Conventionally, to each feed of the focal array there corresponds a narrow beam radiated by the antenna and an area of ground coverage known as a “spot”. It is possible to obtain a radiation of the antenna by multiple beams if the elementary beams are decoupled from each other, the decoupling being either spatial or obtained by the use of orthogonal polarizations or different frequencies between two adjacent beams. The laws of geometry make it possible to project the desired ground coverages into the focal plane of the antenna and to correctly position the phase centre of each primary feed corresponding to each spot. When the coverage is composed of spots regularly arranged on the ground, the offset between two adjacent spots directly determines the space separating two adjacent feeds in the focal plane.
The formation of a large number of contiguous narrow beams implies the fabrication of an antenna comprising a large number of elementary radiating elements, placed in the focal plane of a parabolic reflector. In the case of a conventional antenna in SFPB (Single Feed Per Beam) configuration corresponding to one feed per beam, the volume allocated for the placing of a radiofrequency RF channel intended to perform the transmitting and receiving functions in circular bipolarization is bounded by the radiative surface of a radiating element.
In this configuration where each feed, composed of a radiating element coupled with a radiofrequency channel, forms a beam, each beam formed is transmitted, for example by a dedicated horn constituting the elementary radiating element, and the radiofrequency channel carries out, for each beam, the transmitting/receiving functions in single polarization in a band of frequencies chosen according to the needs of the users. To obtain good radiation efficiency for the spots, the horns of the radiating arrays must enjoy enough space to enable them to be sufficiently directive, in order to illuminate the edge of the reflectors at sufficiently low levels and thus make it possible to limit losses due to spillover. Since the spots are interleaved, the space between two feeds of an antenna may not be compatible with the physical dimensions of the horns to attain the desired radiofrequency performance. For example, this is the case for spot sizes of less than 1°. To solve this problem, three or four different antennas, each producing a third or respectively a quarter of the coverage, are generally chosen. Thus, two adjacent spots of the coverage are not produced by the same antennas. When there is no constraint on the layout of the antenna array, this configuration generally makes it possible to obtain very effective antenna performance. However, when the diameter of the beams diminishes, the geometrical constraints increase and it is not possible to have sufficient space to install each horn despite sharing the coverage over three or four antennas. For very narrow spots of a size between 0.2° and 0.4°, the space allocated to each feed of the focal array becomes very small and the reflector is seen by each feed of the focal array under a sub-tended angle not allowing the feeds to produce sufficient directivity to avoid spillover losses.
A second antenna configuration allowing the forming of a large number of contiguous narrow beams uses a system of two antennas in MFPB (Multiple Feeds Per Beam) configuration using several feeds per beam. Generally, the first antenna Tx operates as the transmitter, the second antenna Rx operates as the receiver, and for each antenna, each beam is formed by combining the signals issued by several adjacent elementary feeds, some of these feeds being re-used to form contiguous beams. A satisfactory radiation efficiency is obtained thanks to the re-use of the feeds, which participate in the formation of several beams, making it possible to increase the radiative surface allocated to each beam and to reduce spillover losses. When the feeds are shared between several beams of the same frequency and polarization, it is possible to create a condition of independence between the beams sharing radiating elements by imposing the formation of so-called orthogonal laws. Orthogonality is achieved by using directional couplers which isolate two-by-two the distribution circuits of the beamforming network BFN which share the same radiating elements. However, the orthogonality constraints provoke a deformation of the radiation patterns of the antennas and an increase in the ohmic losses of the recombining circuits related to the complexity of the distribution circuits. The cumulative losses are often significant, i.e. of the order of 1 dB. Furthermore, it is necessary to limit the complexity of the beamformers to a re-use rate of two radiating elements per spot. This leads to the physical separation of the combining circuits of two adjacent beams by a distance corresponding to two adjacent radiating elements. For spots with an angular offset of between 0.2° and 0.3°, the apparent focal length can be very large, for example of the order of 10 meters. Finally, the re-use of the feeds when forming two adjacent beams presents major drawbacks related to the dimensions of the combining circuits, the weight of the beamformer and the complexity of forming the amplitude and phase laws for each antenna. Indeed, for a re-use of two feeds per polarization, the number of elementary radiofrequency RF channels increases by a factor of greater than four with the number of spots to be formed. Thus, for 100 spots, a number of RF channels greater than 400 radiating elements is required, which necessitates a surface in the focal plane of the order of 500 mm*500 mm. The weight and the volume of the beamformer then become unmanageable.
It is known from patent FR 2 939 971 that a very compact radiofrequency channel can be made using an asymmetric OMT with two branches, associated with an unbalanced branched coupler. This radiofrequency channel operates in dual polarization as transmitter and receiver and comprises radiofrequency components and combining circuits, the dimensions of which do not exceed the horn diameter.